1. Field of the Invention
The present invention relates generally to plasma etching of thin film structures such as microelectronics. More particularly, it relates to a method for shielding substrates from charged particles during plasma etching to provide protection during certain process conditions, or to modulate etching anisotropy.
2. Background of the Invention
Plasma etching is commonly employed in manufacturing microelectronics and micromechanical devices. Plasma etching is used to remove thin films, pattern thin films, or for forming micromachined features. In plasma etching, radiofrequency power is applied to a gas mixture, generating charged particles which aid the etching process. Uncharged reactive particles are also very important to the plasma etching process in that they provide most of the material removal.
In most applications of plasma etching, it is important to control the plasma characteristics so that the plasma has selectivity for certain materials. Also, it can be important in many cases to control the plasma so that it removes material anisotropically or isotropically, as this will greatly effect the shape of etched features. This can be extremely important in manufacturing microelectronics where the shape of an etched sidewall can determine the electrical properties of a device. Also, plasma etch anisotropy/isotropy can be a critical factor in manufacturing micromechanical devices since the etching anisotropy can determine the shape of a final device. It would be a considerable advance in the art of plasma etching to provide a method for tuning the anisotropy/isotropy of a plasma etch process. It would also be an advance to be able to switch between high anisotropy and high isotropy in the same etching system and using the same etching gas mixture. Such capabilities would allow for fabrication of novel structures, and could increase throughput in microelectronics manufacturing.
Another concern with plasma etching is the buildup of thin films on the plasma etch chamber walls. Thin films are often deposited on the walls of a chamber during plasma etching, or during deposition processes. Plasma enhanced chemical vapor deposition (PECVD) of silicon dioxide, for example, typically forms oxide deposits on chamber surfaces far from the target wafer. Also, polymers can be deposited on the chamber walls during plasma etching. Such unwanted thin films on the chamber surfaces can change the electromagnetic properties (e.g. impedance) of the chamber, and thereby alter the amount of radiofrequency energy coupled to the plasma. This results in inconsistent plasma etching characteristics that can reduce manufacturing yield.
In order to prevent changes in chamber characteristics and maintain consistent plasma etching characteristics, the chamber is periodically cleaned of deposited thin films (a process called ‘seasoning’). In seasoning, a special seasoning gas mixture is flowed into the chamber. The seasoning gas mix is formulated to remove films from the chamber walls and restore the chamber to its original condition. Wafers are typically removed during seasoning because the seasoning gas mixture can damage the thin films on the wafer. For example, the unwanted films on the chamber can be the same as the films on the wafer, and exposing the wafer to seasoning will remove the thin films from the wafer as well.
However removing the wafer during seasoning takes time and therefore tends to reduce system throughput. Also, seasoning with the wafer removed can damage the electrostatic chuck since the seasoning plasma can attack the chuck. It would be an advance in the art of plasma processing and chamber seasoning to provide a method for seasoning a chamber with the wafer in-situ while avoiding damage to the wafer. Such an advance would increase manufacturing throughput, and allow for increased control in plasma processing.
Accordingly, the present invention provides an apparatus for tuning or altering the isotropy/anisotropy of plasma etch processes and for allowing in-situ chamber seasoning. Consequently, the present invention provides many advantages such as improved plasma system performance, higher throughput, and higher yield.